In the related art, as a method of controlling a flow rate of a heating medium in a heat source system, for example, a method disclosed in PTL 1 is known. PTL 1 discloses a heat source system including a pump operation state control device which controls a rotational speed of an inverter-driven liquid feed pump, and a flow rate control device which controls a flow rate of a heating medium of each load piping system. In this heat source system, the operation state control device of the liquid feed pump outputs a command for full opening of a valve to a flow rate control valve of the load piping system which becomes a maximum heating medium flow rate, controls the rotational speed of the liquid feed pump so as to obtain a desired flow rate, and controls a flow rate of a different piping system with a small required flow rate by the opening of each flow rate control valve.